In the manufacture of multi-pair telephone communications cables, it has been the usual commercial practice to strand already twisted pairs of conductors helically about a longitudinal axis. Two approaches to the manufacture of such cables are drum-stranding and unit stranding. In the former approach, a cable core is assembled on a strander having a drum in which supply reels of twisted conductor pairs are mounted. The drum is rotated as the conductor pairs are pulled from the supply reels onto a takeup reel so that a unidirectional twist called a stranding lay is imparted to the cable core. A plurality of drums may be placed in tandem to provide a required number of conductor pairs for a particular size cable with successive drums usually driven in opposite directions to apply the conductor pairs in layers in which the directions of the helix as between adjacent layers is reversed to impart desirable electrical characteristics to the cable.
Because of production limitations in cable size imposed by apparatus for drum stranding, unit type cable structures have generally replaced the drum-stranded layer type. In the unit type structure, the cable includes one or more bound, bundles of twisted pairs of conductors which are referred to as units and which are assembled in apparatus called flyer stranders. The stranding is accomplished by paying out the individual twisted pairs of conductors from supply reels mounted in stationary racks through apertured guides or unit faceplates and through stationary unit forming dies onto a reel which is rotated in a cradle to take up the twisted conductor pairs as a flyer bow is revolved about the cradle. In larger size cables, a plurality of units are assembled into a core in a rotating takeup apparatus which is referred to as a cabling machine.
Unit type cables have been formed with a so-called false or reverse stranding lay, that is, a lay that reverses direction periodically along the length of the cable. Individual units are formed by passing pairs through a faceplate which includes a plurality of openings that may be disposed in concentric circles consistent with a layering arrangement of conductor pairs of the cable design. In order to reduce crosstalk between adjacent units in a multiunit cable which is assembled in a cabling apparatus, the faceplate of each unit is oscillated through a predetermined angle in a predetermined length of unit. To maintain the reverse lay in the conductor pairs, they are bound together such as, for example, by a binder tape immediately upon emerging from a unit forming station.
Cable structures which are manufactured by the two aforementioned approaches differ in several important characteristics. The relative position of a conductor pair in one layer of a reverse layer, drum-stranded cable is continuously changing with respect to a conductor pair in an adjacent layer through each stranding lay. However, the relative positions between pairs in adjacent layers in a unit type cable structure remain substantially unchanged. This is of no consequence with respect to crosstalk between conductor pairs in the same layer because that is easily controlled by twist length selection. From the standpoint of crosstalk between conductor pairs, the reverse layer structure is more attractive because pairs exhibiting poor unbalances are not continuously exposed to each other along the length of the cable. Additionally, it has been shown that there is a tendency for conductor pairs to migrate and become displaced from an assigned position in unit type cable structures. This movement seemingly does not occur in reverse layer, drum-stranded cable. Another important difference appears to be that the space per pair or cross sectional area occupied by a pair of insulated conductors is greater for reverse-layer, drum stranded cable than for unit type cable which results in improved mutual capacitance for a reverse layer cable over that for a unit type.
It is beneficial to use the unit type approach with its capability of making larger pair size cables, while incorporating the desirable features of the drum stranding approach. For example, if the electrical characteristics of the unit type cable are satisfactory, the same space per pair with drum stranded cable as a unit type cable may be achieved with a smaller diameter-over-dielectric (DOD) of each insulated conductor. This would permit a reduction in the DOD with an accompanying reduction in the amount of insulation material.
In the prior art, it is known to assemble a plurality of layers, each of which has a plurality of conductor pairs where the conductor pairs in at least one layer are parallel in the form of a helix having a direction of lay reversed periodically along its length and differing from the lay of conductors in an adjacent layer. The cable is made by arranging strand elements in concentric layers, reciprocatingly rotating the layers in alternately opposite directions, bringing the layers together and applying a constraining element around the outer layer. See for example, U.S. Pat. No. 3,187,495. The layers of reverse-oscillated pairs which simulates the effect of reverse layer, drum stranded cable may also be brought together, bound and taken up in a rotating takeup which imparts a stranding lay to the unit. In doing so, the stranding overcomes but is pertubated by the individual reverse twists imparted to each pair by the faceplates.
Problems in systems of the type just described include the rotation of a massive takeup, limitations on the length of cable imposed by the rotation of such a mass, and the inability to test the cable unit until after stranding. What is needed and what is not provided by the prior art is an oscillated lay cable which can be made by methods and apparatus that overcome the aforementioned problems. The sought-after cable should be capable of being made by forming each of the layers of a unit with an oscillated lay and by taking up the units in a non-rotating takeup. Provisions are made for testing the cable during manufacture and before payout to a stranding or cabling machine.